In layers of ferromagnetic transition metals such as Ni, Fe or Co and their alloys, the electric resistance may be dependent on the strength and direction of a magnetic field which passes through the material. The effect which occurs in the case of such layers is known as anisotropic magnetoresistance (AMR), or anisotropic magneto-resistive effect. It is based physically on the different scattering cross sections of electrons with different spin and the spin polarity of the D band. The electrons are therefore referred to as majority or minority electrons. For similar magneto-resistive sensors, a thin film of such a magneto-resistive material with magnetization in the plane of the layer is generally provided. The change in resistance upon rotation of the magnetization with respect to the direction of the current can amount to a few per cent of the normal isotropic (ohmic) resistance.
Furthermore, for some time magneto-resistive multi-layer systems have been known which contain several ferromagnetic layers arranged in a stack, which are separated from each other by metallic intermediate layers and the magnetization of each of which lies in the plane of the layer. The thicknesses of the individual layers are, in this connection, definitely less than the mean free path of the conduction electrons. In such multi-layer systems, a so-called giant magneto-resistive effect or giant magneto-resistance (GMR) can occur now in addition to the anisotropic magneto-resistive effect (see, for example, European Patent No. 483 373). Such a GMR effect is based on the different degrees of scattering of majority and minority conduction electrons at the boundary surfaces between the ferromagnetic layers and the adjacent intermediate layers, as well as on scattering effects within the layers, particularly upon the use of alloys. The GMR effect is, in this case, an isotropic effect. It can be considerably greater than the AMR effect and assume values of up to 70% of the normal isotropic resistance. In similar multi-layer systems which display a GMR effect, adjacent metallic magnetic layers are initially magnetized in opposite directions, a bias layer or bias-layer part being magnetically harder than a measurement layer. Under the influence of an external magnetic field, the initial anti-parallel alignment of the magnetizations can be converted into a parallel alignment. This fact is utilized in such magnetic field sensors.
From German Utility Model No. 93 12 674.3 a magnetic field sensor is known which has sensor elements which are interconnected to form a Wheatstone bridge. These sensor elements have an anisotropic magneto-resistance AMR. In the bridge circuit of such a sensor, the fact can be utilized that, in the individual sensor elements, the magneto-resistive effect of the AMR layers depends on the angle between the magnetization of the corresponding layer and the direction of a current flowing through it. The individual sensor elements can advantageously be so interconnected by suitable structuring to form the bridge that the directions of the current in the two pairs of diagonally opposite bridge elements formed by the two bridge branches form a predetermined angle.
If it were desired to construct corresponding bridge circuits with thin-film sensor elements which have a GMR effect, the problem arises that there is no dependence of the magneto-resistive effect on the direction of the current flow. The change in resistance is therefore the same for all elements in the case of uniform magnetization of the bias-layer parts and of the measurement layers of these elements. Sensor elements with GMR effect could therefore be interconnected to form a Wheatstone bridge only if different directions of the magnetization of their bias-layer parts could be set for the pairs of diagonal bridge elements. With the extremely small dimensions required for such sensors, which dimensions are in the millimeter range or below, no practical possibility, however, was seen up to now of setting different directions of magnetization in the bias-layer parts of adjacent elements in such a narrow space, so that it was not possible up to now to think of producing bridge circuits with GMR sensors on an industrial scale.